1. Field of the Invention
The present invention relates to a wire bonding method known as a method for establishing an electrical connection in producing a semiconductor device, and a wire bonding apparatus used therefor. Further, it relates to a semiconductor device produced using the method and apparatus.
2. Description of the Background Art
Hitherto, in producing a semiconductor device having a semiconductor chip, the wire bonding method has been used for establishing electrical connection between a connection electrode of the semiconductor chip and an external leading electrode on the side on which the semiconductor chip is to be mounted. The material of the wire used for bonding generally contains aluminum or gold as a major component; however, those containing copper as a major component may be used in some cases. Hereafter, in the wire bonding method, the operation of connecting one end of the wire to the first site among the two sites to be connected with the wire is referred to as xe2x80x9cfirst bondingxe2x80x9d, and the operation of connecting the other end of the wire to the second site is referred to as xe2x80x9csecond bondingxe2x80x9d.
Roughly classified, the wire bonding methods can be divided into the ball bonding method and the wedge bonding method.
With reference to FIGS. 10A, 10B, 11, and 12, the ball bonding method will be described. Referring to FIGS. 10A and 10B, a step of forming a ball is performed before the first bonding. Namely, referring to FIG. 10A, a wire 2 is allowed to protrude from a tip end of a bonding tool 1, and a torch electrode 35 is allowed to approach a wire tip end part 3 to generate an arc discharge. This allows the wire tip end part 3 to be instantaneously melted locally with a high temperature caused by the arc discharge, whereby a ball 31 such as shown in FIG. 10B is formed and resolidified in an extremely short period of time. After this ball 31 is pressed onto a site to be joined, a pressure is applied with the bonding tool 1, and a supersonic wave is applied for vibration. Even if an oxide coating film or the like is present on a surface of the ball 31 or on a surface of the site to be joined, the oxide coating film is destroyed by friction accompanying the supersonic wave vibration, whereby the metals are brought into direct contact with each other to generate diffusion for joining. A heat may be applied besides the supersonic wave. For example, the metals are subjected to the supersonic wave vibration while being heated at about 300xc2x0 C. Here, the ball bonding method is generally utilized only for the first bonding, and the second bonding is usually carried out in the same manner as the later mentioned wedge bonding method.
In the ball bonding method, a wire 2 material once melted immediately after the arc discharge resolidifies as the ball 31, whereby a recrystallized region 34 appears at a neck part of the ball 31, as shown in FIG. 11. Such a recrystallized region 34 has a property of being hard and brittle, so that this part cannot be bent to a great extent. Therefore, the configuration of the wire bonding must be designed in such a manner that the wire 2 starts to bend mainly at a part above the recrystallized region 34, as shown in FIG. 12. For this reason, the ball bonding method cannot meet the demand of a so-called xe2x80x9clower loopxe2x80x9d for restraining the loop height H, which is the maximum height of the wire 2 from the joining surface 15.
As a countermeasure for solving this problem, an attempt is made to reduce the recrystallized region 34 by adjusting the amount of impurities mixed into the material of the wire 2; however, another new problem occurs by adjusting the amount of the impurities. Namely, a so-called xe2x80x9csink markxe2x80x9d is generated in which holes appear in the inside of the ball 31, or the joining property is deteriorated, or a brittle alloy layer is liable to be generated.
Further, by the ball bonding method, the ball 31 is deformed to a ball collapse diameter of 60 to 100 xcexcm, which corresponds to about three to four times of the diameter (20 to 30 xcexcm) of the wire 2, for joining. This requires a large area for joining. Furthermore, the diameter of the ball 31 formed as a result of the arc discharge is varied, and it is difficult to precisely control the diameter. For this reason, the configuration must be designed on the basis of the maximum attainable diameter, thereby requiring a further large area. Therefore, the ball bonding method is extremely disadvantageous in satisfying the demand of a so-called xe2x80x9cfiner pitchxe2x80x9d for reducing the pitch of the wire bonding and carrying out the wire bonding at a high density.
A method that compensates for this disadvantage is the wedge bonding method. Referring to FIGS. 13 to 18, the wedge bonding method will be described. In this method, first, a part of the wire 2 is allowed to protrude from the bonding tool 1, as shown in FIG. 13. The process up to this step is the same as that of the aforesaid ball bonding method. A wire bending rod 6 is displaceably placed in the vicinity of this protruding wire tip end part 3. As shown in FIG. 14, the wire bending rod 6 moves to hit the wire tip end part 3 thereby to bend the wire tip end part 3. As a result of this, the wire tip end part 3 is bent along the shape of a pressing surface 14 of the bonding tool 1.
As shown in FIG. 15, the bonding tool descends together with the wire 2, and the bent wire tip end part 3 is pressed onto an external leading electrode 7 which is an object of the first bonding. Here, in the same manner as in the ball bonding method, a supersonic wave is applied while applying pressure with the pressing surface 14. Heat may be applied besides the supersonic wave. For example, supersonic wave vibration is applied while heating at about 300xc2x0 C. As a result of this, the wire tip end part 3 is crushed into a flat shape, as shown in FIG. 15, and is joined to the external leading electrode 7.
Next, as shown in FIG. 16, the bonding tool 1 is moved to a position of the second bond, and is pressed onto a connection electrode 53 of a semiconductor chip 8. The wire 2 follows the movement of the bonding tool 1 from the first bond even if it is not bent by the wire bending rod 6, so that a part of the wire 2 is always sandwiched by the pressing surface 14 and pressed. Here, in the same manner as in the first bonding, a supersonic wave is applied to the sandwiched wire 2 while the wire 2 is pressed by the pressing surface 14. Alternatively, heat is applied besides the supersonic wave. Thus, the wire 2 is joined to the connection electrode 53.
While a wire cutting clamp 9 is in a released state, the bonding tool rises by a length of L, as shown in FIG. 17, and the wire cutting clamp 9 is closed. While the wire 2 is held by the wire cutting clamp 9, the bonding tool 1 rises. Then, the wire 2 is fractured at an end of the region where the wire is pressed into a flat shape by the second bond. As a result of this, the bonding tool 1 rises in a state in which the wire 2 is protruding by the length of L from its tip end, as shown in FIG. 18. Then, the process proceeds to the next operation for the first bonding.
The wire tip end part 3 crushed by the wedge bonding method occupies an elongate shape having its width increased to about twice the diameter; however, the width is small as compared with the area occupied by the ball 31 in the ball bonding method, thereby providing an advantage for the finer pitch. Further, since it is not resolidified after being once melted, the recrystallized region 34 (See FIG. 11) does not appear, thereby providing an advantage for the lower loop.
However, in the wedge bonding method, the bending direction of the wire tip end part 3 is determined by the direction in which the wire bending rod 6 can move, so that the lying direction of the wire pressed onto the joining surface is determined. Therefore, the direction in which the wire bonding can be carried out is limited and, if one wishes to perform wire bonding in a different direction, the wire bonding apparatus or the work piece must be rotated, thereby leading to low productivity.
Therefore, an object of the present invention is to provide a wire bonding method and a wire bonding apparatus being advantageous to the lower loop and the finer pitch and having a high productivity.
In order to achieve the aforesaid object, a wire bonding apparatus according to one aspect of the present invention includes a bonding tool having a pressing surface for pressing a wire onto a surface to be joined and a wire feeding hole being open to the pressing surface; a wire feeding means for feeding the wire to an outside of the bonding tool through the wire feeding hole; and an attracting and holding means for applying an attraction force to a wire tip end part that is protruded from the wire feeding hole, thereby to bend and hold the wire tip end part towards the pressing surface.
By adopting the aforesaid construction, the wire tip end part can be bent and held by the attraction force applied by the attracting and holding means without the use of an external member such as a rod. The wire can be joined by pressing the wire together with the bonding tool onto the surface to be joined and treating it in the same manner as in the wedge bonding method.
The aforesaid invention preferably includes an attraction force distribution changing means for changing a distribution of the attraction force of the attracting and holding means thereby to bend the wire tip end a part in a desired direction. By adopting this construction, the wire tip end part can be bent and held in a direction in which the wire is desired to be joined. Therefore, it can be applied to wire bonding in a semiconductor device in which the joining direction of the wire changes depending on the electrode.
In the aforesaid invention, the attracting and holding means preferably includes a magnet disposed to apply a magnetic attraction force from the pressing surface to the wire tip end part. By adopting this construction, the wire tip end part containing a magnetic substance to be capable of being attracted by magnetic force can be attracted towards the pressing surface by magnetic force.
In the aforesaid invention, the magnet is preferably an electromagnet. By adopting this construction, the presence or absence of the magnetic line of force can be switched by whether the electric current is passed or not, whereby an operation of generating or releasing the attraction force can be carried out easily.
Further, a wire bonding apparatus according to another aspect of the present invention includes a bonding tool having a pressing surface for pressing a wire onto a surface to be joined and a wire feeding hole being open to the pressing surface; a wire feeding means for feeding the wire to an outside of the bonding tool through the wire feeding hole; a plurality of attracting and holding means for applying an attraction force to a wire tip end part that is protruded from the wire feeding hole, thereby to bend and hold the wire tip end part towards the pressing surface; and an attracting and holding means selecting means that can bend the wire tip end part in a desired direction depending on which of the plurality of attracting and holding means is operated.
By adopting the aforesaid construction, the wire tip end part can be bent and held by the attraction force applied by the plurality of attracting and holding means without the use of an external member such as a rod. The wire can be joined by pressing the wire together with the bonding tool onto the surface to be joined and treating it in the same manner as in the wedge bonding method.
In the aforesaid invention, the attracting and holding means preferably include a plurality of suction holes disposed to surround the wire feeding hole and each being capable of generating a negative pressure. By adopting this construction, the wire tip end part can be attracted by the negative pressure to be bent and held even if the wire is made of a material that cannot be attracted by magnetic force.
In the aforesaid invention, the suction holes are preferably disposed in the pressing surface. By adopting this construction, the wire tip end part can be attracted towards the pressing surface and can be held in a state in which the wire tip end part is adhering to the pressing surface, whereby the wire tip end part can be pressed by the pressing surface with certainty.
Further, in order to achieve the aforesaid object, a wire bonding method according to the present invention is a wire bonding method using a bonding tool having a pressing surface for pressing a wire onto a surface to be joined and a wire feeding hole being open to the pressing surface, wherein the method includes a wire feeding step for feeding the wire from the wire feeding hole; an attracting and holding step for applying an attraction force to a wire tip end part that is protruded from the wire feeding hole, thereby to bend and hold the wire tip end part towards the pressing surface; and a joining step for pressing and joining the wire tip end part onto a surface to be joined by the pressing surface.
By adopting this method, the wire tip end part can be bent and held by the attraction force applied in the attracting and holding step without the use of an external member such as a rod. The wire can be joined in the joining step in the same manner as in the wedge bonding method.
In the aforesaid invention, the wire tip end part is preferably bent in a desired direction by changing a distribution of the attraction force in the attracting and holding step. By adopting this method, the wire tip end part can be bent towards the side where the attraction force is strong, and can be held on the pressing surface.
In the aforesaid invention, the wire tip end part is preferably bent in a desired direction by switching which of a plurality of attracting and holding means is operated in the attracting and holding step. By adopting this method, the selection of the direction in which the wire tip end part is to be bent can be carried out by selecting the means to be operated from among the plurality of attracting and holding means, so that the selection can be carried out easily and with certainty.
In the aforesaid invention, the attracting and holding step preferably includes applying a magnetic attraction force to the wire tip end part. By adopting this method, the wire tip end part containing a magnetic substance to be capable of being attracted by magnetic force can be attracted towards the pressing surface by magnetic force.
In the aforesaid invention, the magnetic attraction force is preferably generated by an electromagnet. By adopting this method, the presence or absence of the magnetic line of force can be switched by whether the electric current is passed or not, whereby an operation of generating or releasing the attraction force can be carried out easily.
In the aforesaid invention, the attracting and holding step preferably includes applying a negative pressure to the wire tip end part. By adopting this method, the wire tip end part can be attracted by the negative pressure to be bent and held even if the wire is made of a material that cannot be attracted by magnetic force.
Further, a semiconductor device according to the present invention includes a wiring line subjected to wire bonding by making use of a bonding tool having a pressing surface for pressing a wire onto a surface to be joined and a wire feeding hole being open to the pressing surface, feeding the wire through the wire feeding hole, applying an attraction force to a wire tip end part that is protruded from the wire feeding hole, thereby to bend and hold the wire tip end part towards the pressing surface, and pressing and joining the wire tip end part to the surface to be joined by the pressing surface.
By adopting this construction, the wire can be joined in a desired direction, so that the wire can be disposed by effectively utilizing the space, eliminating the need for a large area such as a ball in the joining site. This is advantageous for the finer pitch. Further, since the balls are not formed, the recrystallized region is not generated, so that a small and thin semiconductor device can be produced which is advantageous for the lower loop.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.